Many industrial processes require the intake or exhaust of large quantities of gas. Such industrial processes include power generation, heating, ventilation, and air conditioning (HVAC), forced-air cooling and fume extraction, as well as in processes, especially combustion processes, of the oil and gas industries.
Transport of such gas is usually achieved by flowing the gas through ducting of appropriate dimension and construction from the source of the gas to a location at where it is required to be exhausted or used. In the case of intake of gas, for example atmospheric air, the gas is transported from an intake vent, possibly provided with suitable filtering means, to a location at which the gas is used or required, via a suitable intake duct. In the case where gas is to be exhausted, the gas is transported from the location at which it is produced, possibly via various treatment processes such as filtering and scrubbing, to an exhaust vent at which the gas is to be exhausted, via an exhaust duct.
Firstly, the processes which use or generate the gas, such as combustion processes, may generate large quantities of undesirable noise. Further, where the rate of transport of gas is to be high, the transport of gas can be associated with the generation of undesirable noise. Also, the means by which the gas is drawn into or drawn out of the location where the gas is produced or consumed, for example fans and the like, can generate substantial amounts of undesirable noise themselves. Transport of this noise via the gas in the duct to the surrounding environment is undesirable. Particularly, health, safety and environmental considerations, as well as applicable laws and regulations, often restrict the noise level to which plant operators, local workers, local residents or wildlife is exposed. Therefore, there is a need to provide reliable means of reducing the noise level associated with gas intake and gas exhaust processes.
It is known in the art to provide gas transport ducts with elements which are able to absorb a portion of the acoustic energy present in the gas flow through the duct, thereby to reduce the noise level associated with the gas transport process. Such elements are often referred to as sound suppression elements.
One example of a technique for sound suppression in a gas transport duct includes the positioning of resistive sound-absorbing elements in the duct. These elements are made of a material which presents resistance to the propagation of the sound wave, for example by being composed of a mass of fibres or having a large number of inter-connected pores. Interaction of the sound wave with this material absorbs acoustic energy from the sound wave and converts it to another form of energy, typically heat. Portions, such as sheets, of such resistive sound-absorbing material may be applied to the inner walls of the duct, so as to absorb acoustic energy from gas passing through the duct. Often, the materials used as resistive sound-absorbing materials, such as rock wool or glass wool, are relatively fragile and can be eroded by the passage of high velocity gas, especially passing at elevated temperatures. Therefore, it is known to secure the resistive sound-absorbing material against the wall using a layer of perforated material, such as steel mesh, which is more resistant to erosion by the flow of gas. Such a configuration is disclosed in British patent application publication GB 2 122 256 A.
Resistive sound-absorbing material typically absorbs efficiently only frequencies at which the thickness of the absorbent material is greater than a quarter wavelength of the sound. For frequencies below this limit, the sound-absorption process is far less efficient. Further, the effectiveness of the sound-absorbing process increases with the proportion of the internal surface of the duct exposed to the gas flow which is provided with resistive sound-absorbing material, as well as the length of duct over which the gas flow can interact with the resistive sound-absorbing material. Therefore, in order to suppress across a broad frequency band efficiently, a large thickness of resistive sound-absorbing material must be placed such that a relatively large proportion of the wall area is provided with the resistive sound-absorbing material. This tends to decrease the internal cross-sectional area of the duct.
However, as the internal cross-sectional area of the duct is decreased, the gas flow velocity increases for a given gas flow rate. Increasing the internal cross-sectional area of the duct to counter this effect reduces the sound absorption performance. Therefore, in order successfully to use resistive sound-absorbing material to achieve adequate sound suppression across a broad range of frequencies without decreasing the cross-section of the duct available for gas flow, the outer dimensions of the duct must, both in terms of cross-section and in terms of length, be increased. However, such an approach results in larger, heavier and correspondingly more expensive gas handling equipment.
In some circumstances, it is known to install resistive baffles inside a gas flow duct. Such baffles are installed within the cross-sectional flow path of the duct, and can either run fully across a cross-sectional dimension of the duct, essentially splitting the duct into two flow passages on either side of the baffle, or can be provided with a relatively smaller length than the cross-section of the duct into which they are installed so as to be arranged in a central space of the duct. The former is known as a splitter baffle, the latter is known as a brick baffle, sometimes also known as a bar baffle. The general plan view of such a resistive baffle, whether a brick baffle or a splitter baffle, is shown in FIG. 25.
In FIG. 25, baffle 2500 provides a mass of resistive sound-absorbing material 2510 of sufficient thickness, length and cross-sectional area to achieve a desired broadband reduction of unwanted acoustic frequencies. Since baffle 2500 is to be placed in the gas flow inside the duct, baffle 2500 is also provided with a cap portion 2520 arranged to face the oncoming gas flow, and being configured to have a convex surface facing the oncoming gas flow so as smoothly to divert the gas flow to either side of the baffle. Cap portion 2520, by diverting the gas flow to either side of baffle 2500, functions firstly to reduce the impact of the baffle on smooth gas flow through the duct and secondly to reduce the possibility that the resistive sound-absorbing material 2510 will be eroded by the oncoming gas. Cap portion 2520, in the case of a splitter baffle, may have a constant cross-section running the entire vertical height of the baffle, which is perpendicular to the plane of the page. In some configurations, cap portion 2520 may be substantially semi-circular, and may smoothly join with essentially parallel sides of the resistive sound-absorbing material 2510. In some configurations, the mass of resistive sound-absorbing material is shielded by a screening material such as a mesh or fabric, which may define part of the outer surface of the baffle. In such a configuration, the screening material may be considered to form part of the resistive sound-absorbing material.
In the configuration of a brick silencer, FIG. 25 may represent not only the plan view, but also the elevation view, and cap portion 2520 may be in the form of a hemispherical dome, and again may be formed to join smoothly with parallel circumferential sides of resistive sound-absorbing material 2510 in the case where the baffle 2500 has an overall circular cross-section viewed in the direction of gas travel. In other configurations, baffle 2500 may have a generally square or rectangular configuration in the direction of gas travel, and the shape of cap portion 2520 may be appropriately selected.
At the other end of baffle 2500 to cap portion 2520, a fairing portion 2530 is provided to reduce further the impact of baffle 2500 on the smooth flow of gas through the duct. The fairing portion is constructed with a slight taper relative to the parallel walls of the resistive sound-absorbing material 2510 for reducing the impact on the flow of gas past the baffle.
An alternative technique for sound suppression to the resistive sound-absorbing element is the so-called reactive, sometimes termed reflective, sound-attenuating element. Such reactive sound-attenuating elements involve passing the gas flow past a geometrical feature such as a depression, channel or cavity, the geometry of which causes the propagation of a sound wave at a characteristic frequency in opposite direction and/or in opposite phase to the sound waves propagating in the transported gas. This can be achieved in one approach by changing the acoustic impedance of the gas flow duct, for example by expanding the cross-sectional area, so as to generate a reflected wave at the characteristic frequency in counter-phase to the incoming acoustic energy. Alternatively, this can be thought of as altering the acoustic resistance coupled to the sound-wave radiating source.
Alternatively, the gas flow may be split into two gas flows, travelling along two paths having different lengths, which are re-combined so that the interference of the sound waves at the exit of each path results in cancellation of the acoustic energy. Here, the path-length difference defines the characteristic frequency of the reactive sound-attenuating element. Further alternatively, a resonant cavity may be coupled to the flow path, either directly or via a connecting channel, the dimensions and geometry of the resonant cavity, as well as the area and length of the tube communicating between the resonant cavity and the flow path, being selected so that the waves generated in the resonant cavity when excited by the incident sound waves cancel out a desired portion of the noise in the gas flow. Such a resonant cavity can be thought of also as a trap for the acoustic energy in the gas flow, wherein the resonant mode or modes of the cavity, being a characteristic frequency of the resonator, absorb desired portions of the acoustic energy spectrum. The characteristic frequency may be a fundamental frequency of the resonator, or may be a harmonic frequency of the resonator.
One well-known configuration of resonant sound-absorber is the Helmholtz resonator, which consists of a large chamber connected by a narrow tube to the gas flow path. The characteristic frequency is defined by the geometry of the chamber. Another form of resonator is the quarter-wavelength resonator, which consists of a closed pipe extending from the gas flow path and having a length of approximately one quarter the intended wavelength of the characteristic frequency to be suppressed. A variant of the quarter-wavelength resonator is the so-called half-wavelength resonator, consisting of a pipe which, rather than having straight sides, tapers to a point and has a length of half the wavelength of the characteristic frequency which is intended to be suppressed. Further, there is the eighth-wavelength resonator, consisting of a pipe similar to the quarter wavelength resonator, but which is divided into an open-ended portion and a closed-ended portion which communicate with each other, the depth of which from the channel is one-eighth of the wavelength of the characteristic frequency which is to be suppressed.
Examples of each of these reactive sound-absorber types are shown in FIG. 24. Configuration A shown in FIG. 24 is the so-called impedance mismatch or expansion resonator, where the diameter of the duct is expanded to produce a reflection wave. Sometimes, perforated material covers the expansion region to maintain stable gas flow past the expansion region. Configuration B shown in FIG. 24 is a Helmholtz resonator, wherein a resonant cavity is connected by a short tube to the gas flow duct. Configuration C shown in FIG. 24 is the quarter-wave resonator, in the form of a short, closed pipe. Configuration D shown in FIG. 24 is the half-wave resonator, in the form of a tapered, closed pipe. Configuration E shown in FIG. 24 is the eighth-wave resonator, in the form of a divided, closed pipe. Configuration F shown in FIG. 24 is a side-channel resonator, sometimes termed the Herschel-Quincke tube, having the form of a short side-channel having a desired path length difference compared with the gas flow in the duct.
Although reactive sound-attenuating elements can be effective at suppressing a well-defined frequency and its harmonics, reactive sound-attenuating elements lack the broad-band sound-absorbing properties of the resistive absorber. Furthermore, providing such sound-attenuating elements in the walls of a duct again entails increasing the duct dimensions for a given desired gas flow rate and requires costly engineering of the walls of the duct to provide the desired resonant sound-absorbers at appropriate locations in the duct. Finally, providing such sound-attenuating elements in the walls of a duct is ineffective in suppressing higher frequencies which propagate in the duct at higher-order modes of the duct, since the acoustic energy associated with such frequencies propagates along the duct in mode patterns which are distant from, and thus interact less, with the walls of the duct.
It has therefore been proposed, in International patent application publication pamphlet WO 98/27321 A, to provide a modular duct in which i) ducting sections having reactive sound-attenuating elements integrated into the walls and ii) ducting sections having resistive sound-absorbing elements integrated into the walls are sequentially arranged. While such an approach can achieve broadband sound suppression with the resistive absorber and sound-absorption at specific frequencies with the resonant absorbing sections, such a configuration requires an increase in the external cross-sectional area of the duct for a given internal cross-sectional area, and thus inevitably results in a requirement for large, heavy and expensive ducting.
Furthermore, since the frequency spectrum associated with a gas transport duct results from a combination of the rate at which gas is transported along the duct, the gas-producing or gas-consuming equipment which is connected to the duct, the presence of intermediate gas treatment units such as filters, and the length and geometry of the duct itself, the provision of reactive and resistive sound-absorbers as part of the duct requires complex and expensive custom engineering work in designing the duct to meet the specific requirements of the project.
Despite the above-proposed techniques for control over unwanted noise in gas transport ducts, there is a need for sound suppression apparatus which is able to suppress a broad range of unwanted frequencies, for example high levels of targeted low-frequency control as well as good broadband performance, at high efficiency and which is cost-effective to manufacture and install.